Waveform: scrolling amplitude envelope (song-style) tied to the speed slider

Replaces the fast instantaneous oscilloscope with a mirrored filled waveform
that scrolls over time like DJ-software waveforms; scroll rate follows the
visual speed control.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/output.js

@@ -249,7 +249,7 @@ djv.onControl(async (m) => {
     case 'recStart': startRecording(); break;
     case 'recStop': stopRecording(m); break;
     case 'gain': audio.gain = m.value; break;
-    case 'speed': speed = m.value; break;
+    case 'speed': speed = m.value; audio.scrollRate = m.value; break;
     case 'bandGain': audio[m.band + 'Gain'] = m.value; break;
     case 'outputDevice':
       try { await audio.setOutputDevice(m.deviceId || ''); }